Lidar Mean Power Reduction

Abstract

A laser radar device adapted to ensure that the output from the laser is eye-safe. A means for applying spatial dither to the output of the laser source, such as a moveable optical arrangement. This causes the point of focus of the transmitted beam to traverse a target area by small amounts, reducing the overall radiation exposure at any particular point of focus, but having negligible impact on wind speed measurement for example. Alternative arrangements for ensuring eye-safety include periodically reducing the laser power density, gating the output or altering the focussing of the transmitted beam.

1 . A laser radar device comprising a laser source and a means of applying spatial dither to the output of the laser source. 
     
     
         2 . A method of operating a laser radar device comprising the steps of generating a beam of radiation from a laser source and applying spatial dither to the beam as transmitted from the device so as to move the point of focus. 
     
     
         3 . A method as claimed in  claim 2  wherein the point of focus of the transmitted beam is moved such that the point of focus of the beam traverses an area in space greater than the beam cross section area. 
     
     
         4 . A method as claimed in  claim 2  wherein the maximum amount of spatial dither applied is less than 20 times the beam diameter at the point of focus, or less than 10 times the beam diameter at the point of focus or less than 5 times the beam diameter at the point of focus. 
     
     
         5 . A laser radar device comprising a laser source and output optics further comprising a mean power reducing means. 
     
     
         6 . A laser radar device as claimed in  claim 5  wherein the mean power reducing means is arranged to periodically reduce the laser power density of the output beam of radiation. 
     
     
         7 . A laser radar device as claimed in  claim 6  wherein mean power reducing means periodically intensity modulates the beam produced by the laser source. 
     
     
         8 . A laser radar device as claimed in  claim 7  wherein the mean power reducing means prevents emission of an output beam of radiation outside a measurement period. 
     
     
         9 . A laser radar device as claimed in  claim 7  wherein the mean power reducing means intensity modulates the laser output beam by direct control of the laser. 
     
     
         10 . A laser radar device as claimed in  claim 7  wherein the mean power reducing means comprises an intensity modulator acting on the output of the laser source. 
     
     
         11 . A laser radar device as claimed in  claim 5 , wherein the mean power control means is adapted to alter the focussing of the transmitted beam. 
     
     
         12 . A laser radar device as claimed in  claim 11  wherein the mean power control means is arranged to defocus the transmitted beam when not making a measurement. 
     
     
         13 . A laser radar device according to  claim 5 , wherein the mean power reducing means comprises a means of applying a spatial dither to the transmitted beam so as to move the point of focus. 
     
     
         14 . A laser radar device as claimed in  claim 13  wherein the maximum spatial dither applied is greater than or equal to the beam diameter at the point of focus. 
     
     
         15 . A laser radar device as claimed in  claim 13  wherein the maximum amount of spatial dither applied is less than 20 times the beam diameter at the point of focus, or less than 10 times the beam diameter at the point of focus or less than 5 times the beam diameter at the point of focus. 
     
     
         16 . A laser radar device as claimed in  claim 13  wherein the mean power reducing means is adapted to apply continuous spatial dither. 
     
     
         17 . A laser radar device as claimed in  claim 13  wherein the mean power reducing means is adapted to apply periodic spatial dither. 
     
     
         18 . A laser radar device as claimed  claim 13  wherein the mean power reducing means is adapted to apply spatial dither on a timescale such that the point of focus is dithered at least once in each measurement time. 
     
     
         19 . A laser radar device as claimed  claim 13  wherein the rate of application of spatial dither is such that the point of focus is varied from a first position to a second non-overlapping position. 
     
     
         20 . A laser radar device as claimed in  claim 13  wherein the means for applying spatial dither comprises a moveable optical element. 
     
     
         21 . A laser radar device as claimed in  claim 20  wherein the means for applying spatial dither comprises a rotatable transmissive element. 
     
     
         22 . A laser radar device as claimed in  claim 20  wherein the means for applying spatial dither comprises a moveable mirror arranged in the optical path to direct the transmitted beam. 
     
     
         23 . A laser radar device as claimed in  claim 20  wherein the means for applying spatial dither comprises means for moving the fibre optic cable relative to some transmit optical elements. 
     
     
         24 . A laser radar device as claimed in  claim 1 , wherein the radar device is a staring laser radar device or a stepped staring laser radar device. 
     
     
         25 . A laser radar device according to  claim 1 , comprising fibre optic cable for guiding radiation from the source to the device output.